The production of multicolor images by the so-called silver-dye-bleach process is based on the use of a photographic element comprising a plurality of superposed silver halide emulsion layers, each of which contains a bleachable dye of a color complementary to the color of that portion of the spectrum to which the respective silver halide layer is sensitive. In the practice of the silver-dye-bleach process, the element is exposed, developed and subsequently treated in a special acidic bleach bath wherein the dyes are rendered colorless in proportion to the metallic silver produced during development. After removal of the residual silver and silver halide by silver bleach, fixing, and washing, a positive dye image is obtained. A common silver-dye-bleach process as described above suffers from the disadvantage that the dyes incorporated in the light-sensitive silver halide emulsion layers absorb a sufficient amount of the exposing light to reduce the effective speed of the film material.
Various methods have been employed to overcome this disadvantage. One such method has consisted of placing the dyes in separate coating layers below the silver halide layers. While such a transposition of the dyes has improved the effective speed of the film somewhat, it has also led to unsharp images due to the increased thickness of the film which has brought about a scattering of the exposing light and created a longer diffusion path along which the dye bleach catalyst has to travel from the silver to the dye. Other attempts to overcome the above-described shortcoming, such as false sensitization or dyeing of the film after exposure, have been equally unsatisfactory. The replacement of the preformed azo dye by various forms of colorless precursors of the dyes, such as, for example, nitroso compounds or azo couplers, has also proven to be of only limited advantage inasmuch as the conversion of the precursors to their respective dyes requires at least one additional processing step. A fourth known method of enhancing the speed of conventional silver-azo-dye-bleach materials has been that of incorporating into the material special azo dyes which can exist in differently colored forms, depending on the pH of the surrounding medium. During the exposure, such pH-sensitive dyes are of a color suitable to transmit light of those wavelengths to which the layers containing the dyes, or the layers lying beneath them, are sensitive. Again, such a method is undesirable because an additional pH-changing treating bath is required in order to shift the hue of the dyes to that which is desirable in the finished image, and because these special dyes generally have poor stability upon prolonged keeping.
It is also known in the art that colorless dye precursors can be used instead of the bleachable dyes. For example, U.S. Pat. No. 3,503,741 to Wilson et al discloses a silver-dye bleach process wherein colorless tetrazolium salts are utilized as precursors for the bleachable dyes. This material alleviates the speed loss as described, but the salts can chemically desensitize the silver halide grains during pre-exposure storage.
There is, of course, a need for improved photographic elements and processes wherein the elements contain colorless dye precursors which do not interfere with the desired image-making chemistry and wherein the photographic elements can have very high effective exposure speeds.